In the optical communications space, various techniques are used to synthesize optical communications signal for transmission. The term “optical” refers to electromagnetic radiation in both the visible and non-visible portions of the electromagnetic spectrum. An optical signal encodes information in high and low amplitude states or phase changes of a channel of electromagnetic radiation. A term “channel” refers to a single wavelength of electromagnetic radiation or a narrow band of electromagnetic radiation centered about a particular wavelength. For example, a high amplitude portion of an optical signal can represent a logic bit value “1” and a low amplitude portion of the same optical signal can represent a logic bit value “0,” or vice versa. The optical signal can be transmitted over a waveguide, such as an optical fiber, or over free space.
Direct and indirect modulation are two processes by which optical signals can be generated. With direct modulation, the drive current of an electromagnetic radiation emitting device is modulated, such as modulating the drive current of a laser or a light-emitting diode (“LED”). Unfortunately, direct modulation of radiation emitting devices has a number of drawbacks. One problem with direct modulation is that the optical power source must be built in the same technology as the modulators. Cost-effective modulators typically need to be built in silicon and silicon is an indirect hand-gap material. Hence, the power of silicon lasers is fundamentally limited which limits their utility to very short distances. In addition, direct modulation can shift the output wavelength of an optical signal, an effect called “chirp,” which adds to chromatic dispersion.
Indirect modulation, on the other hand, is performed with two separate component types: 1) an unmodulated optical power source channel and 2) a modulator which selectively couples a portion of the optical power source energy to a modulated data channel, or selectively removes the light from the optical power source to encode the data. The selective coupling or removal modulator functionality provides a simple on-off keying data encoding mechanism. Indirect modulation allows both the optical power source and the modulators to be built with separate technologies that is most efficient for its intended purpose. Indirect modulators typically provide faster modulation rates than direct modulation and typically do not alter the wavelength of the modulated optical signal.
Whether the optical signals are produced using direct or indirect modulation, wavelength division multiplexing is typically used to combine multiple optical signals for transmission along a single waveguide. However, in recent years, the demand for fast and energy-efficient modulation and wavelength division multiplexing systems has increased in order to keep pace with the increasing demand for high-speed data transmission between communicating devices.